The present invention generally relates to semiconductor devices and more particularly to a semiconductor optical device that shows a non-linear optical response.
In relation to the high-speed optical telecommunication systems and optical digital processors, the non-linear semiconductor optical devices are studied intensively. In the non-linear semiconductor optical devices, the optical property such as the transmittance or reflectance is changed in response to the irradiation of a control optical beam. Thus, the device is suitable for constructing optically triggered optical switches, optical logic processors or optical modulators.
In the Laid-open European patent application EP 0 358 229 and in the corresponding U.S. patent application Ser. No. 404,958 as well as in the U.S. patent application Ser. No. 758,857 that is a CIP application of the foregoing Ser. No. 404,958, the inventors of the present invention have proposed a so-called TBQ (tunneling bi-quantum well) structure for the non-linear semiconductor optical device.
In the TBQ structure, a quantum well layer is provided together with a pair of barrier layers for forming discrete quantum levels such that the carriers are excited to the quantum levels upon interaction with an incident optical beam. In response to the excitation, the incident optical beam is absorbed by the quantum well layer. In order to accelerate the recovery of the original optical state after interruption of the incident optical beam, the TBQ structure uses a second quantum well layer adjacent to the barrier layer with a quantum level formed at a level lower than the quantum level of the first mentioned quantum well. Further, the thickness of the barrier layer is set small enough such that the carriers, particularly the electrons, can cause a tunneling through the barrier layer. Thereby, the free electrons that are excited in the first quantum well layer upon absorption of the incident optical beam immediately escape to the second quantum well layer, and the quick recovery of the original optical state such as the original optical absorption or original refractive index is guaranteed.
FIG. 1 shows the band structure of a conventional TBQ device.
Referring to FIG. 1, the TBQ device has an active layer 14a that is sandwiched by a pair of barrier layers 14b. The thickness of the active layer 14a is set small enough such that discrete quantum levels of electrons and holes such as E.sub.1 and H.sub.1 are formed therein. In other words, the active layer 14a forms a quantum well. There, the quantum well layer 14a absorbs the incident optical beam having a wavelength corresponding to the energy gap between the quantum level E.sub.1 and H.sub.1 by causing an excitation of the electrons to the quantum level E.sub.1. Further, the incident optical beam is absorbed by creating excitons that exist at the energy level slightly lower than the quantum level E.sub.1. The excitons thus produced are further decomposed by releasing electrons and holes to the quantum levels E.sub.1 and H.sub.1.
In the TBQ structure of FIG. 1, the thickness of the barrier layer 14b is set small enough such that the electrons and holes that are excited to the quantum level E.sub.1 can escape from the quantum well layer 14a freely. Further, in order to accelerate the escaping of the electrons from the quantum well layer 14a, another quantum well layer 14c is provided adjacent to and in contact with the barrier layer 14b. Typically, the second quantum well layer 14c has a thickness substantially larger than the thickness of the first quantum well layer 14a and is characterized by quantum levels E.sub.2 and H.sub.2 that are substantially lower than the corresponding quantum levels E.sub.1 and H.sub.1 of the first quantum well layer 14a. There, the electrons escape from the first quantum well layer 14a to the second quantum well layer 14c through the barrier layer 14b and the unwanted dwelling of the carriers in the first quantum well layer 14a is avoided. It should be noted that the carriers that are dwelling on the excited state impede further absorption of the incident optical beam. In order to recover the original state after the interruption of the incident optical beam, one has to remove the excited carriers as fast as possible.
In the TBQ structure shown in FIG. 1, it should be noted that the energy gap between the quantum levels E.sub.2 and H.sub.2 is much smaller than the energy of the incident optical beam. Thereby, there occurs a problem in the device of FIG. 1 in that the incident optical beam is absorbed inevitably by the second quantum well layer 14c. In other words, the non-linear semiconductor optical device of FIG. 1 tends to show a relatively poor signal-to-noise ratio.
The TBQ device of FIG. 1 has another problem in that the electrons and holes that have escaped from the first quantum well layer 14a to the second quantum well layer 14c tend to dwell in the layer 14c. For example, this problem becomes conspicuous when the incident optical beam is supplied in the form of high speed optical pulses. Particularly, there is a tendency that the electrons are preferentially accumulated in the second quantum well layer 14c because of the smaller effective mass as compared with the holes. On the other hand, the holes tend to be accumulated in the first quantum well layer 14a. When this occurs, the energy level of the second quantum well layer 14c may shift gradually relative to the first quantum well layer 14a. Ultimately, the quantum level E.sub.2 may become substantially identical with the quantum level E.sub.1. When such a situation occurs, the removal of the carriers from the quantum well layer 14a is no longer effective and the response of the non-linear semiconductor optical device is inevitably deteriorated.